Your total testosterone came back in the normal range. You feel like garbage anyway: tired, no drive, foggy. Your doctor shrugs. You suspect something is being missed.
Free testosterone is often the answer. Only a tiny slice (roughly 1 to 3 percent) of circulating testosterone is unbound and able to enter cells and do anything. The rest is held by carrier proteins, mostly SHBG. When SHBG is high (which gets more common with age, hyperthyroidism, oral estrogen, liver disease), your total looks fine but the bioavailable fraction is genuinely low. That gap is what makes the difference between a lab printout and how you feel.
One catch. The cheap "direct" or "analog" free testosterone immunoassay that most labs run is not reliable. The Endocrine Society recommends against it. Order equilibrium dialysis (gold standard, accurate, slower) or calculated free testosterone (derived from total T, SHBG, and albumin). If your report says "free testosterone (direct)", the result is closer to noise than signal.
What free testosterone measures
Testosterone in blood exists in three states:
- SHBG-bound: tightly bound to sex hormone-binding globulin, biologically inactive (~40–60%).
- Albumin-bound: weakly bound, dissociates readily, considered bioavailable (~40–50%).
- Free: unbound, fully active (~1–3%).
"Bioavailable testosterone" = free + albumin-bound. Some labs report this in addition to free.
Free testosterone matters most when SHBG is unusual:
- High SHBG (aging, hyperthyroidism, oral estrogen, anorexia, liver disease) → total testosterone looks low even when free is fine.
- Low SHBG (obesity, type 2 diabetes, hypothyroidism, exogenous androgens, Cushing's) → total testosterone looks normal even when free is low.
In either situation, total alone misleads. Free testosterone is the answer.
Free testosterone reference ranges
| Demographic | Low | High | Unit |
|---|---|---|---|
| Adult men 19–39 | 9 | 30 | ng/dL |
| Adult men 40–59 | 7 | 25 | ng/dL |
| Adult men 60+ | 5 | 22 | ng/dL |
| Adult women (premenopausal) | 0.1 | 6 | pg/mL |
| Postmenopausal women | 0.1 | 2 | pg/mL |
| Endocrine Society hypogonadism cutoff (men) | 0 | 65 | pg/mL |
Reference ranges depend heavily on assay. The values below assume equilibrium dialysis or calculated free T.
- Adult men 19–39: roughly 9–30 ng/dL (90–300 pg/mL).
- Adult men 40+: declines with age; lower limit drops to ~7 ng/dL (70 pg/mL) by age 70.
- Adult women: 0.1–6 pg/mL by equilibrium dialysis. Cycle and age-dependent.
- Postmenopausal women: 0.1–2 pg/mL.
Different units across labs are common, pg/mL vs. ng/dL vs. pmol/L. Always interpret against the reporting lab's reference range, not memorized cutoffs.
The Endocrine Society guideline uses calculated free testosterone below 65 pg/mL (or below the lab's lower limit of normal) as the diagnostic threshold for hypogonadism in men with normal SHBG.
What high free testosterone means
Causes overlap with high total testosterone but with extra emphasis on SHBG-driven elevation:
- Exogenous testosterone or anabolic steroids: most common cause. Free T rises proportionally with total.
- Low SHBG state: obesity, insulin resistance, type 2 diabetes, hypothyroidism. Total T may be normal but free T is high. This is the typical pattern in PCOS in women.
- Polycystic ovary syndrome: in women, free testosterone is often more sensitive than total for confirming hyperandrogenism.
- Adrenal or gonadal androgen-secreting tumor: rare; usually total is also dramatically elevated.
- Congenital adrenal hyperplasia: late-onset; pair with 17-OH progesterone.
In a man with high free testosterone and unexplained suppressed LH/FSH, exogenous androgen use is overwhelmingly the cause.
What low free testosterone means
The interesting cases are men with normal-looking total testosterone but low free. Causes:
- Aging: SHBG rises with age, sequestering more testosterone. Free testosterone declines faster than total. Many older men with "normal" total testosterone are functionally hypogonadal.
- Hyperthyroidism: drives SHBG up, lowers free fraction.
- Liver disease: chronic hepatitis, cirrhosis raise SHBG.
- Anorexia or severe caloric restriction: high SHBG, low free.
- Estrogen exposure: oral estrogen therapy, some forms of hormone therapy, exposure to high environmental estrogens, all raise SHBG.
- Anticonvulsants: phenytoin, carbamazepine raise SHBG.
- True hypogonadism: both total and free are low. The standard story.
In women, low free testosterone is rarely the explanation for symptoms. Female testosterone biology is poorly characterized and assays are imprecise at the lower end of the female range.
Calculated vs. measured free testosterone
Three methods exist:
- Equilibrium dialysis (gold standard). Physically separates free from bound. Accurate, expensive, slower turnaround. Worth ordering when the answer matters, fertility workup, ambiguous total testosterone, confirming hypogonadism.
- Calculated free testosterone. Uses total testosterone, SHBG, and albumin in a formula (Vermeulen equation). Excellent agreement with dialysis when inputs are accurate. The pragmatic best choice for most outpatient workups.
- Direct (analog) immunoassay. Cheap, widely available, and unreliable. The Endocrine Society explicitly advises against using direct free testosterone for diagnosis. If your lab report says "free testosterone (direct)" or "free testosterone (analog)", re-order the right test.
Free testosterone, like total, follows a strong diurnal rhythm, peak in the morning, trough in the afternoon. Same rules apply: test before 10 a.m., fasting preferred, repeat if borderline.
Track this biomarker over time in AskAnything.health — upload your lab results and see trends at a glance.
When to act on free testosterone
- Confirming hypogonadism in older men: total testosterone borderline, free testosterone clearly low → real deficiency.
- Investigating PCOS: free testosterone is more sensitive than total for hyperandrogenism in women.
- Discordant total and free: points to SHBG-driven story; check SHBG, thyroid (TSH), liver function (ALT, AST).
- Free testosterone clearly elevated with suppressed LH/FSH in a man: exogenous androgen use until proven otherwise.
- Don't act on a "direct" free T: re-test by dialysis or calculation.
This information is for educational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider about your lab results.
Tests that complete the picture
- Total testosterone: always pair; the two together tell the bioavailability story.
- SHBG: required for calculated free T; independently informative.
- Albumin (albumin): third input for calculated free T; useful as marker of nutritional status.
- LH / FSH: primary vs. secondary localization.
- Prolactin: rule out prolactinoma in unexplained hypogonadism.
- Estradiol: particularly relevant in obesity (high aromatization) and in men on testosterone therapy.